Avoid unsupported util.isError function removed in Node.js 24

Fixes #46.

Co-authored-by: Timo Tijhof <krinkle@fastmail.com>

Author: Aiosa

index.js

@@ -11,7 +11,6 @@
 
 // Nodejs libs.
 var spawn = require('child_process').spawn;
-var nodeUtil = require('util');
 var path = require('path');
 
 // The module to be exported.
@@ -52,7 +51,7 @@ util.callbackify = function(fn) {
 // Create a new Error object, with an origError property that will be dumped
 // if grunt was run with the --debug=9 option.
 util.error = function(err, origError) {
-  if (!nodeUtil.isError(err)) { err = new Error(err); }
+  if (!(err instanceof Error)) { err = new Error(err); }
   if (origError) { err.origError = origError; }
   return err;
 };